Topographical models of geographical areas may be used for many applications. For example, topographical models may be used in flight simulators and for planning military missions. Furthermore, topographical models of man-made structures (e.g., cities) may be extremely helpful in applications such as cellular antenna placement, urban planning, disaster preparedness and analysis, and mapping, for example.
Various types and methods for making topographical models are presently being used. One common topographical model is the digital elevation map (DEM). A DEM is a sampled matrix representation of a geographical area which may be generated in an automated fashion by a computer. In a DEM, coordinate points are made to correspond with a height value. DEMs are typically used for modeling terrain where the transitions between different elevations (e.g., valleys, mountains, etc.) are generally smooth from one to a next. That is, DEMs typically model terrain as a plurality of curved surfaces and any discontinuities therebetween are thus “smoothed” over. Thus, in a typical DEM no distinct objects are present on the terrain.
One particularly advantageous 3D site modeling product is RealSite® from the present Assignee Harris Corp. RealSite® may be used to register overlapping images of a geographical area of interest, and extract high resolution DEMs using stereo and nadir view techniques. RealSite® provides a semi-automated process for making three-dimensional (3D) topographical models of geographical areas, including cities, that have accurate textures and structure boundaries. Moreover, RealSite® models are geospatially accurate. That is, the location of any given point within the model corresponds to an actual location in the geographical area with very high accuracy. The data used to generate RealSite® models may include aerial and satellite photography, electro-optical, infrared, and light detection and ranging (LIDAR). Another similar system from Harris Corp. is LiteSite®. LiteSite® models provide automatic extraction of ground, foliage, and urban digital elevation models (DEMs) from LIDAR and IFSAR imagery. LiteSite™ can be used to produce affordable, geospatially accurate, high-resolution 3-D models of buildings and terrain.
Another advantageous approach for generating 3D site models is set forth in U.S. Pat. No. 6,654,690 to Rahmes et al., which is also assigned to the present Assignee and is hereby incorporated herein in its entirety by reference. This patent discloses an automated method for making a topographical model of an area including terrain and buildings thereon based upon randomly spaced data of elevation versus position. The method includes processing the randomly spaced data to generate gridded data of elevation versus position conforming to a predetermined position grid, processing the gridded data to distinguish building data from terrain data, and performing polygon extraction for the building data to make the topographical model of the area including terrain and buildings thereon.
One difficulty in generating automated topographical models is generating realistic looking foliage, and particularly trees. This is because geospatial modeling data is often captured from above the geographical area of interest from an airplane or satellite, and thus the raw image data may not include data points corresponding to the trunks of trees, only the leaves/needles thereon. Thus, when a three-dimensional (3D) digital elevation model (DEM) of the scene is generated, it may only include the crowns of trees and not the trunks.
Various approaches have been used for attempting to determine the location and heights of tree trunks from collected image data. For example, in an article by Pyysalo et al. entitled “Reconstructing Tree Crowns from Laser Scanner Data for Feature Extraction,” ISPRS Commission III, Symposium 2002 Sep. 9-13, 2002, Graz, Austria, 2002, a study to reconstruct single tree crowns from laser scanner data to use the obtained vector model for feature extraction is described. As part of the reconstruction methodology, an estimate of the location of the tree trunk was calculated from crown points as a mean value of x and y coordinates weighted with point height. The tree trunks were considered as straight lines from the top of the tree to the surface of the digital terrain model. This reference line was then used to help estimate an average distance of points from the trunk at different heights.
Another approach is described in a paper by Yu et al. entitled “A GIS-based Forest Visual Simulation System,” Proceedings of the Third Int'l Conf. on Image and Graphics, IEEE, 2004. This paper reports on a visual simulation system that supports GIS-based modeling and real-time rendering of forest scenes. The system uses geometric models or templates of trees that are automatically generated according to inventory database and pre-designed template models. Some similar approaches use stand-in templates that are manually drawn (i.e., “billboards”) that are layered in a 3D model to provide the appearance of a forest.
Despite the existence of such approaches, further techniques may be desirable for simulating realistic tree structure, including trunks and branches, in 3D geospatial models. In particular, it may be desirable to provide realistic representations of actual trees as they would appear in a geospatial scene to preserve spatial relationships and accuracy in a corresponding geospatial model.